Automatic radio control device



Feb. 23, 1943. R. ,w. GUDIE AUTOMATIC RADIO CONTQL DEVICE Filed Jan. 18,1941 2 Sheets-Sheet 1 los .20u

Q mm; Gomk 1f INVENToR. RAY W. Guo/E I A 7' Tok/vt K R. w. GUDIEAUTOMATIC RADIO CONTROL DEVICE Filed Jan. 18, 1941 2 Sheets-Sheet 2 wwwINVENTOR. AY W'. Guo/E .idf/zul; y

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was EG MWWNLN Patented Feb. 23, 1943 Unire. e

mrs

TENT @FFEQE Ray W. Gudie, Los Angeles, Calif, assignor to Leo M. Harvey,La Canada, Calif.

Application January 18, 1941, Serial No. 375,006

(Cl. Z50-11) 4 Claims.

This invention relates to automatic radio navigational equipment, andparticularly to equipment wherein comparison must be made between somecharacteristic, other than the relative amplitudes, of two radio signalswhose strength at the point of reception diners. Examples of suchdevices are radio pilots or automatic steering con-v trols such as thosedisclosed by Goble and Phillips in copending application, Serial No.314,854, led January 20, 1940, Patent No. 2,247,294 granted June 24,1941, blind landing systems such as that of Goble, Phillips and Thacker,Serial No. 325,632, filed March 23, 1940, Patent No. 2,264,056 grantedNovember 25, 1941, and various other applications where like problemsmust be met.

Each of the inventions specically mentioned has in common with theothers that its operation depends upon the comparison of somecharacteristic of two radio signals other than their absolute amplitudesat the point of reception, and that the comparison may most convenientlybe made by comparing the modulation components of the signals.

In the present specication my invention is shown as applied to the radiocontrol device disclosed by Goble and Phillips in their application,Serial No. 314,854 above mentioned. Here the comparison is to be madebetween the direction of' reception of two radio waves. This directionof reception, however, is reflected in the percentage of modulation ofthe two signals afterv these signals have been operated upon by theequipment', and the most feasible method of so comparing the signals isto mix them and to operate the indicating or control device by theresulting amplitude of the modulation component, this nents at the pointin the system where they are detected or de-modulated may be a constantwhich is independent of the strength of the received Waves. In otherwords, what is requiredis an automatic volume control which will feed anabsolutely uniform orflat input to the detector.

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Furthermore, one of the characteristics of a device of the typementioned is that the reference signal is that picked up on a loopantenna, which antennae have the characteristics that their receptivityis proportional to the sine of the angle between the axis of the loopand the direction from which the wave-is received. Thisl being the case,an automatic volume control which operated in inverse proportion to thesignal fed to the receiver by the loop would be useless, since thetendency of the automatic volume control would be to suppress the veryvariation in signal intensity which the device utilizesas a measurementof angle.

In the aforementioned application a separate receiver was used for thesole purpose of actuating the automatic volume control. One of theobjectsv of this invention to reduce the complexity of the apparatus byincorporating the automatic volume control into the main amplifier whichoperates upon the signal to be measured. Other objects areY to providea, simple type of automatic volume control which Will s'o operate upon asignal as to amplify it in rigorously inverse proportion to its el'dintensity at the point of pickup; to provide a radio direction iindercircuit of the right-and-left indicating type which will give a maximummodulation component for a giveny angle between the zero axis of theantenna system and the direction of reception of the wave; to provide asystem which will operate over a maximum variation in intensity of thereceived wave, so las t'o give a direct comparison between the l azimuthof immediately adjacent and far distant f stations; to provide `a systemin accordance with the Goble and Phillips invention abovel mentionedemploying a minimum number of antennae; and to provide a radioAdirectioniinder system wherein the reception. characteristics may bemade of substantially perfect card'ioid pattern, giving a maximum ofsensitivity.

Referringto-the drawings:

, Fig. 1 isa schematic diagram of a radio direction iinder embodyingmyinvention;

Fig. 2 is a schematic diagram showing two such direction nders asapplied to the radio control device ofthe Goble and Phillipsapplication, Serial No. 314,854;

Since this device comprises twol substantially identical radio direction'nders, plus certain equipment which is common to both, the duplicatedevices--are'indicated in the drawings by the YdistinguishingcliaractersfA' and B, and a single descriptionwill`suffice 'for both. Like reference characters are applied to likev partsof both receivers.

Each of the two radio direction finders comprises a loop antenna I, theaxis of which is the axis of reference for the direction of the receivedsignals. The loop is provided with a grounded center tap 2, and acrossits two ends a coupling coil 3 is connected. This type of loop circuitmakes it posible to balance the loop circuit accurately to ground, thusexcluding all vertical antenna component from the signal deliveredthereby, and giving it a reception characteristic of true sine-waveform, i. e., making the polar diagram of said characteristic accuratelyrepresent the theoretical figure-eight pattern.

The coupling coil 3 is the primary of a radiofrequency transformer,whose secondary coil 4 is tuned to the frequency of the received signalby means of a variable condenser 5. One end of the' coil 4 is grounded.The other end connects to the control grid 6 of a radio-frequencyamplifying tube, in this case a pentode 1` In order to simplify thedrawings, the conventional filament circuit of this tube, as Well. asthose of the tubes later to be described, are omitted. Bias for the tube1 is provided by a conventional cathode resistor Ill connected betweenground and the separately heated cathode II. The plate I2 is fed throughan inductor I3 and a by-passed voltage regulating resistor I4 'from acommon plate bus I5 from which all tubes (except as particularly pointedhereinafter) are supplied. This bus I5 connects to the positive terminalof a suitable source I1, indicated as a battery, although a vibratoryrectifier or a motor generator may be substituted. The screen grid I9 ofthe tube 1 connects through a resistor to a screengrid bus 2|, which isfed through a voltage divider comprising resistors 22 and 23 from thesame source I1.

The plate end of the coil I3 connects through blocking condensers 24, 24in parallel to the two grids 25, 25 of a double triode 21. The plates29, 29 of the double triode 21 connect to opposite ends of a balancedcoupling coil 30 having a center tap connecting through a bypassedresistor 3I to the plate bus I5. A bypassed cathode resistor 32 connectsfrom the cathode 33 to ground. u

In operation the grids 25, 25' are alternately swung negatively belowcutoff by a suitable voltage wave, for example, such as is generated bythe oscillator indicated by the general reference character O andcomprising the equipment shown within the dotted outline indicated bythis. character. The single oscillator feeds both receivers A and B, andits primary requirement is that the waveform generated thereby should besymmetrical. The frequency developed by it is a matter of choice. It ispreferred that it operate somewhere in the range'between 100 and 200cycles, but there is no theoretical limit tothe frequency which may beused; it is merely convenience which places this frequency within thelower audio range. n

In order to obtain the accurately symmetrical waveform desired I preferto utilize a double triode which has its two grids. 31,. 31crossconnected to the two plates 33', 39 respectively through condensers40, 40"., A similar condenser 4I connects the two plates, which are fedthrough leads 42 from; the source I1 in. a manner later to be described.'IheA grid-31 connects through a resistor 43, "which, may have a valuein the neighborhood of 1A megohm, and connects througha second resistor44, of the order of 40,000 ohms, to ground. A small condenser, (e. g.,.01 microfarad) 45 connects from the junction of the resistors 43 and 44to one end of a third resistor 41 of, say, 100,000 ohms resistance, theother end of which is also grounded. The ungrounded end of this resistorconnects through a lead 49 and a series-resistor 50 with the grid 25. Anexactly similar network, whose elements are represented by the samereference characters distinguished by accent, connects from the grid 31to ground and to the grid 25 of tube 21. A condenser 5I having acapacity of the order of .01 microfarad connects between the junctionsof resistors 43 and 44 and resistors 43 and 44'.

It will be recognized that separate triodes could be substituted for thedouble triode 35, but the double triode is preferable because the twosets of elements are more likely to be exactly matched. It is alsoimportant that the values of the resistors and condensers in theoscillator network be exactly matched, but it is not important that theyhave the exact values here assigned to them, since changes in theirvalues will merely vary the frequency of oscillation by varying the timeconstants of the resistive-capacitive circuits which they form. Increasein either resistance or. capacitance will lower the frequency, whiledecrease in these values will raise it. The waveform generated by thisoscillator is quite accurately sinusoidal; it is not necessary, however,that sine waves be used as long as symmetry of waveform is preserved.The present oscillator is used because with it it is easy to maintainsuch symmetry in practice.

It has already been stated that the wave generated by the oscillator Odrives the grids of the tube 21 alternately below cutoff, so that whilethe oscillator O is in operation only one-half of the tube 21 isoperating at any one time. The radio frequency applied to the two grids25, 25 is the same, since the same circuit drives them in parallel. Thesame waves therefore appear in amplified form on the two plates 29, 29alternately, and ysince these plates are connected to opposite ends ofthe coil 30, the two halves of this coil alternately carry the amplifiedsignal. The coil 30 lis coupled to an antenna coil 52 and a mixing coil53, and the phase of the signal transferred to these two coils by thecoupling is therefore reversed with the reversals'of potential from theoscillator. Y

- The antenna coil 52 connects between ground and an antenna 56 which iscommon to -both receivers A and B, through a separate resistor 51 foreach receiver. The antenna 55 may be a whip,

or a horizontal wire'antenna, but whatever the form taken it should besubstantially without directional characteristics and should beeffectively a vertical antenna if` the device is to be used fordirection finding with vertically polarized waves. The resistance 51 isused both to render the antenna aperiodic and to make the signaltransferred from the antenna to the coil 52 equal to that similarlytransferred from the coill 30. Perhaps a more accurate method of statingthis would be to say that the signal transferred to the mixing coil 53bythe antenna should be equal to the value of the signals transferred tothe same coil lfrom the coil 30 fromazimuths of 90 degrees and 270degrees with respect to the axis of the loop I'. The optimum conditionis that the 'vertical antenna effect from` the antenna 56 should beequal to the maximum loop effect. The ampli er tubes 1 andZfIxbringthe-level of .theloop signal up to a. point` where itis-comparable .-inintensity to the signal received from the .Verticalantenna; instead of being so small thatthe recepek tivity of al fewinches of antenna mayI be enough to give asignal equal to the'loopand'theadjust-- ment of the antenna may thus be so critical as to becomealmost impossible. If the antenna-isV connected as here shown thesignals derivedv therefrom may be added to or subtracted from thecomponent of signal from the" loop, even though that component may bemany times as great as the true loop effect current component. Byproperly adjusting the values of the resistor 51, and the number ofturns in the coil 52, with respect to the loop signal as delivered tothe coil 30, the current appearing in the mixing coil may be'madeproportional to lisinl p, where p" is the azimuth from which the signalis received on the loop, and the sign i is dependent upon which of thetwo grids 25, 25 is operating below cutoff.

The signalappearingin the-coil 53 will in any event be proportionaltoAisin p, where A may haveany value, but theadvantage of the presentarrangement isk that it permits making A accurately equal to 1, whichgives -a cardioid reception pattern and maximum sensitivity foranydirection finder of the general type here under consideration.

It should be pointed outthat while it isdesirable that the verticalantenna signal from the coil 52 should equal the maximum loop signal,this is not a necessary condition but is merely an optimum one. For thedevice to function properly, however, the verticalantenna signal must beat leastas great as the loop `signaki. e., the vertical antenna signalmay be larger but1 must not be smaller` than that from the loop.Furthermore, the ratio of vertical antenna signal to loop signal shouldbe the same for both direction finder A' and direction finder B inorder-.to achieve maximum accuracy, and should-'bear very approximatelythe same ratio inany case.

Themixingcoil 53 is tuned by a-variablecondenser 54, and its lowpotential endconnectstoan A. V.. C. bus 55 througha by-.passedretardingresistor 58. The mixed. signals from thecoil 53 are: fed to the control"grid 59 of a variable-mu rst detector or frequency changer-tube` B0.This tube is shown as a hexode, having a screengrid 6| which derives itspotential from the bus 2| through a resistor 62, and a modulator gridv63 which is connected to ground througha biasing resistor 64 and to aheterodyne oscillator through a coupling condenser 65. 'Ihe heterodyneoscillator may be of any approved form;-A a s shown it comprises-atetrode 61 having a grid coil-69r tuned by avariablecondenser1-0and-coupled to anun-v tuned plate coil- 1|'. The methodsof applyingthe requisite potentials upon the` various` electrodes-areconventional.. hence are not described herein.` detail'.

The plate.12;of'the.t1.ib`e` uconnectsto'the pri-V mary of a tunedYintermediate frequency; transformer 13, the primary of which connects tothe plate bus through a by-passed resistor 14 while the secondaryconnects tothe A. V. C. bus 55, through another by-passed'resist'or 15.The secondary feeds two variable-mu tubes'11 and 19 in parallel. Thecontrol` grid 80.of tube 11 is fed4 directly, but the gridA 8| of thetube 19 is fed through av blocking condenser-@Land is-connect. ed toground througha grid'leak; 8 3.

Tube 19 feeds the automatic volume control.- Being a Variable-mu tubeits amplification depends upon its grid bias, which is controllablethrough a variable cathode resistor 84. Its plate feeds a tunedintermediate frequency transed; Thev low tension end of this meshconnects to ground through a resistor and also to the grid 91of the'trio'd'e' portion ofthe tube through a' coupling' condenser 99. Stillanother resistor |00y connects fr'om the junction of resistorsV 93 and95' to the A. V. C. bus 55, which is grounded through'a condenser |0`|.The grid 91 is' biased by a battery |02 through a grid resistor |03 andthe' plate |04 of the triode portion of the tube 9i* feeds an audio'monitoring circuit including telephones cr loud` speaker |05 through aconventional resistance-capacity network |06.

Ashas been pointed out, the tube Tris identical with thetube 19, anditfeeds a tunedintermediate frequency transformer '81' and thence a duplexdiode-triode detector 90" through circuit' elements which are identical'with those coupling the tubes 19 and 90 with the exception that theconnection from' the detector' circuitl to the A. V. C'. bus is omitted.

The output circuit |06" of tube 9'0" is also identical with that of tube90, but instead of feeding the potentiometer |01 and monitoring phonesit feeds. through a filter circuit |09 to a reversing switch ||0`connecting the circuits where the. signals from receivers A and' B arecombined, as will be hereinafter described. Before so describing them,however, we will consifder the. operation of the A. V. C. circuit.

It has been shown that the signal delivered to the first. detector andmodulator tube 60 comprises a component delivered from the verticalantenna56 plus an amplified component from the loop |,y that the loopcomponent is never greater than, the' vertical antenna component, andthat the loop component` is alternately added to andi subtracted from`the vertical antenna component with a periodicity determi-nedfby theoscillator O. Since the WaveA from the-oscillator Gis-symmetrical theincrease in amplitude of the combined componentsy duri-ng' the periodwhen they; are additive isfe'qual intime andin amount to the decrease inamplitude during the" half cycle when the two are subtractive; i. e.,when integrated over anyv number of complete cycles the averageintensity ofthe radio-frequency'signalfed to the modulator tube 60 is aIconstant depending` purely upon the receptivity -of the'ver'ticalantenna and. the intensityv of` the received wave. This-wouldv-not, holdtrue if the loopv component were ever permit-ted to` become greater thanthe vertical antenna component, for if this 'occurred the result wouldbe ov,erf-modulation,v

and a double frequency modulation component would appear in the mixedsignal. Under the specified conditions, however", the4 average signal:is unaffected bythe percentage to whichitis tensity of the heterodyneoscillation from the.

tube 61, multiplied by the amplication factor of the tube 60. Theheterodyning oscillation may readily be kept substantially constant, butif not it is of no particular importance, since its amplitude appears asa product in the output of the tube and the effect of a change inamplitude of the oscillator has the same eiect as a change in theintensity of the received wave, although under ordinary conditions thepercentage change due to change in oscillator amplitude will be muchsmaller than that due to change in received Wave intensity. Theamplified intermediate frequency wave is then fed to tube 19, which hasa nearly constant amplication factor under ordinary conditions, i. e.,when its amplification has once been set by means of the cathode biasresistor 84.

The diode comprising cathode 9| and anode 92 of the tube 90 thereforeoperates upon and rectiies a signal which is directly proportional tothe signal fed to tube 19 times its amplication factor, and the currentwhich the diode permits to flow therethrough is proportional to thepositive halves of the Waves, such diode detectors being linear incharacteristics. The intermediate frequency component of this current isfiltered out and grounded in the network comprising resistor 93 andcondensers 94, and the potential applied across resistor 95, andconsequently the current owing therethrough, are therefore proportionalto the envelope amplitude of the signal delivered from tube 19; i. e.,this potential and current have a D.C. component plus a modulationcomponent which again are proportional to Ai-sin p. The same potentialappears across the resistor |05, and through this resistor it chargesthe condenser Illl. The time constant of this resistorcondensercombination being greatly in excess of the period of the modulationcomponent, the mean potential appearing across condenser IDI to groundis directly proportional to the average intensity of theisignaldelivered by the tube 19; i. e., it is directly proportional to thecarrier or unmodulated component of the intermediate frequency Wave.

This average potential across condenser IDI is applied to the A, V. C.,bus and, further ltered by resistors 58 and 15 and their respectivebypass condensers, appears upon the control grids of tubes 60 and 'llrespectively. Both of these are variable-mu tubes, and the effect of anincreased amplitude delivered by tube 60 is therefore to increase thenegative bias upon this tube and hence tends to cut down its output.Obviously, however, it cannot maintain the output of tube 60 constant,since an increased output is necessary in order to increase the negativebias. The effect, so far as this tube is concerned, is therefore simplyto reduce the eiect of changes in received `signal intensity, as is thecase in the ordinary A. V. C., and not to eliminate them entirely.

The same change in. bias, however, which is effective on tube 60 lisalso. eiective on tube 11, and tube l1 has a characteristic curve of thesame shape as that of tube 19. In the tubes best adapted for the presentpurpose, for example,y the tubes known in the trade as 6SK7, the shapeof the grid-plate curve is logarithmic or substantially so, that is tosay, the slope of the curve relating plate current to grid voltage inthe tube is proportional to the plate current.

Mathematically, the Eg-Ip characteristic of thev tube may be expressedby the equation Ip=KeMEg, where K and M are constants and e is theNapierian base.

When a sine wave signal of amplitude E is applied to the grid of such atube, the output signal current Is has an amplitude which isproportional to where B is the grid bias voltage. this may be expressed:

l eM? K being a constant which takes into account the constant Kpreviously introduced, the form factor of the output wave, and thefactor 2 in the denominator of the expression first given.

A number of things may be learned from the form of the expression. Oneof these is that the quantity in parentheses is a constant for anySimplifying,

ygiven value of signal, irrespective of the grid bias,

and therefore if the same signal be fed to two such tubes as that underconsideration the outputs of those tubes will bear a constant Iratio,irrespective of signal amplitude. Another thing 'shown by the equationis that the amplication amplitude, and that therefore the bias on a selfbiased tube will increase with increased signal,

thus tending to compensate in part for the increase in amplication withamplitude. Finally, the equation shows that if the output signal currentIs is to be a constant,

1 iM-6W must` equal l e MBI: '6MB But the bias voltage B is itself anexponential quantity, proportional to where R is the resistance of theresistor 95, and the factor ofproportionality is not a constant.

as-the tube mais self-biased, and its amplification factor varies withamplitude, as has beenshown above.

practice, however, the vnecessary relationship obtains if certainprecautions be observed. Only a moderate proportion-of thepossiblemaximum output signal of tube 'II should be usedless.4 than half in anyyevent androne-third or even less is preferable. The-proportion of thecurve used Awill-decrease as the bias appliedthereto by .tube `I9.increases. The latter tube should be so self-biased .as not Lto overloadon strong signals. The quantities involved. are not critical, and usingthe fSSK'I amplifier and -a fGSQ-'P detector, proper results can beobtained if the A. V..C. resistor 95 be given a-value of r500,000 ohmsand `the-variable cathode resistor84: almaximum value `of 10,000 ohms.

-Usingthese values, a receiver of this type has `given a sensiblyconstant output between signal strengthsof 100and80,000microvolts permeter.

It is obviously possible to .make the amplification offtube 119 so greatAthat output decreases with increased input, but if adjustment be madein the laboratory, by means of the cathode resistor `81|, until averystrong input signal gives the` same reading as a minimum signal of about100 MV/M, the desired results will be obtained.

By the method thus described ithas-been found possible to maintain theoutput soynearly constar-rt that it'has been impossible te detect anydiierencein signal strength'between'a weak station in NSanjDiegoand apowerful station in Los Angeleswhen a plane in which the system wasinstalled was immediately above the powerful station.

VIt Awill be seen that as Vfar as the linearity of output `is concernedwithin the operating range ofx the device it is unimportant whether ornot the A. y'V.C.-volta .ge be applied to the frequency changer tube6-0, butthat so applying it increases greatly the rangeof operation.

Returningnow to the description of the apparatus, each ofthe receiversAvandiB'feeds one of two-balanced primaries I II I I I cfa transformerI|g2 whose center tapped secondary I-I 3 feeds, in push-pull, the ,gridsof a double triode |I4. The signal fed Ato this tube is the resultant ofthetwo modulating components from the two receivers A andgB. vThesekcomponents both being derived from the oscillator O ar either exactlyin phase or :180 degrees out of phase, depending upon Ythe azimuth ofthe received signalA and vthe settings of the reversing switches I I.-These signals may, therefore, be cumulative, may balance out entirely,or may have a nite resultant which is less than the greater of the twosignals taken alone and whose phase depends upon which signal ispreponderant. This resultant signal is fed through a second push-pulltransformer I I5 and a double triode amplifier I I1 to an outputtransformer having a push-pull primary II9 and a single secondary |20which feeds the grids of two output tubes I2I, I2I' in parallel. Thesetubes may be either of the high vacuum or the grid-glow type, and intheir output circuits are connected relay coils |22, I 22', whoseoperation is the primary purpose of the entire device. Each of thesecoils is shunted by a condenser |23, whose purpose is to flatten out themodulating component.

The plate circuits of the tubes I2 I2 l are supplied with alternatingcurrent derived from the oscillator O. The two plates 39, 39 of theoscillator tube 35 are connected through the lead 42 `to the twov endsof a primary coil |25. A center tap t2?! on this coil is. connectedtothe` source Il tosupply the plate voltage for the tube 35.` A push-pullsecondary |29, coupled to the primary |25, feeds a pair of power ampliertubes |30, which inturn feed a transformer I3I, and the two ends of thesecondary |32 of this transformer connect through the relays |22, |22 tothe tubes |2I, |2I' respectively.

It will be vseen that by this arrangement the plates of the two tubes I2|, I2I are fed with alternating current of thesame frequency as thatfedto the grids of the two tubes, and that the platevoltage will ybe inphase with the grid voltage on one of these tubes and out of phase withthe grid voltage of the other, depending upon the phase of the resultantcurrent from the two-receivers. The grids of the two tubes I2I, 2I'thereforeswing in potential together. VWhen both grids are negativeneither tube will carry current. When both-are positive the plateof onetube is positive and vthat o f the other negative, and only that tubewhose plate is positive will carry current. One and one only oftherelays |22,` I 22 will therefore operate, and which one does this isdependent upon the phase of the mixedsignal delivered by the transformerI I2.

With `the relays |22, |22' connected to control the operation of anairplane, the course of the airplanewill therefore depend upon'thesetting of the switches IIU, but will'be independent vof the proximityof the controlling station.

Sincethe system under discussion is designed to hold -an airplane upon adirect course xed by the line between two controlling stations,irrespective of cross winds, and since this requires that the plane headinto the cross winds in order to maintain its true course, thisindependence of signal strength is very important. In a system wherein aAsingle station only is used for guidance the absolute value of themodulation component of a direction finder is unimportant, since bothright and left indications are derived from the 4Same source and aretherefore of equalintensity at any moment, and will always balance outwhen the Nplane is on course. Where, however, a nite signal derived fromone sourceis balanced against a iinite ,Sgnalfrom another, the relativemagnitude of the vtwo modulation components becomes as important astheir phase. I 'ests of the-present system have proved thai'l itscontrol of volume or amplitude is suiiicientto yield the desired -resuit-to an even greater degree than might be expected, for on a recentseries of nights onfthe Los Angeles-SanlDiego course above mentioned,covering an air-line distance of Well over a hundred miles, the maximumdeviation from a xed point of reference along the course was less thanfifty feet ina series of two nights in each direction. It is obvious,that in a test such as this the plane is subjected to cross winds fromopposite sides as it traverses the course in the opposite directions,and, further, that the toe and heel guiding stations are interchanged.Since a constant wind blows from the Pacic obliquely across this coursesuch a result indicates practically perfect operation.

I claim:

l. A radio direction nder system comprising an antenna system includinga directional receiving element having a null reception axis xed inazimuth, and a non-directional receivingelement, providing respectivedirectional and non-directional signal components, and having areceptivity with respect to radio signals from various azia pair ofvariable-mu amplifier tubes having grids connected'in parallel foractuation by the modulated signals, a separate detector fed by each ofsaid amplifier tubes, means for applying a bias to the gride of one ofsaid amplifier tubes which is proportional to the mean output current ofY the detector fed by the second of said ampliiier tubes andself-biasing means for the grid of said second amplifier tube, wherebythe mean intensity of the signal fed to the detector of said firstamplifier tube remains substantially a constant while its modulationpercentage varies with the azimuth of the received signal.r

2. A radio direction finder system comprising an antenna systemincluding a directional receiving element having a null reception axisfixed in azimuth, and a non-directional receiving element, providingrespective directional and nondirectional signal components, and havinga receptivity with respect to radio signals from various azimuthssubstantially proportional to Aisin p, Where A is not less than one, andp is the angle between the null reception axis of said directionalreceiving element and the direction from which the radio wave isreceived, means for periodically reversing the phase of one component ofsuch signals with respect to the other, to modulate such signals to apercentage represented by the absolute value ofV sin p a pair ofvariable-mu amplifier tubes having grids connected in parallel foractuation by the modulated signals, a separate detector fed by each ofsaid amplifier tubes, means for applying a bias to the grid lof one ofsaid amplier tubes which is proportional to the mean output current ofthe detector fed by the second of said amplifier tubes, means forbiasing the grid of said second amplier tube to control the amplicationthereof, and means for varying said last mentioned bias to so controlthe amplification of said rst mentioned tube as to render it inverselyproportional to the amplitude of the signal fed to both tubes.

3. A radio direction finder comprising an antenna system including meansfor producing from a received signal a reception component independentof the direction of reception and a component varying with the directionof reception with respect to said system, said independent componentbeing at least as great as the maximum value of said varying component,means for periodically reversing one of said components With respect tothe other, an amplier coupled to operate on both of said components,said amplifier comprising a pair of vacuum tubes each having alogarithmic grid-plate characteristic and having control grids connectedin parallel, means for varying the grid bias of one of said tubes tovary the ampliiication factor thereof, means for rectifying the A. C.output of said last mentioned tube, means for rintegrating saidrectified output, means for applying the potential of said integratedoutput to bias the control grid of the other of said tubes to maintainconstant the A. C. output thereof irrespective of the intensity of theinput signal thereto, a detector connected to respond to the output ofsaid second mentioned tube, and a Work circuit fed by said detector.

4. A radio direction finderA comprising an antenna system includingmeans' for producing from a received signal a reception componentindependent of the direction of reception and a component varying withthe direction of reception with respect to said system, said independentcomponent being at least as great as the maximum value of said varyingcomponent, means for periodically reversing one of said components withrespect to the other, a heterodyne frequency changer operativelyconnected to said antenna system; an intermediate frequency ampliiierfed by said frequency changer, said amplier comprising a pair of vacuumtubes each having a logarithmic grid-plate characteristic and havingcontrol grids connected in parallel, means for varying they grid bias ofone of said tubes to vary the amplication factor thereof, means forrectifying the A. C. output of said last mentioned tube, means forintegrating said rectied output, means for applying the potential ofsaid integrated output to bias the control grid of the other of saidtubes to maintain constant the A. C. output thereof irrespective of theintensity of the input signal thereto, a detector connected to respondto the output of said second mentioned tube, and a work circuit fed bysaid detector.

- RAY W. GUDIE.

